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Input lag is the amount of time that passes between sending an electrical signal and the occurrence of a corresponding action—for example, the delay between pressing a key on a keyboard and a letter appearing on the monitor.
In video games, input lag refers to either the delay between the television or monitor receiving a signal and its being displayed on the screen (see display lag below), or the delay between pressing a button and seeing the game react.
In electronic hardware development, input lag is the delay between an electronic input signal being generated (pressing a button as above) and processed (I/O ports have been read, and memory updated to reflect the state of the input). In this field, the phenomena detailed below are referred to as output lag. More formally, the terms input latency and output latency are also used. Due to gamers being largely unaware of the phenomenon of input lag at this low level, the terms input lag and output lag became confused over time, and the above description, where input and output lag are combined into a singular phenomenon known as input lag, has become popular.
Overall, the correct terminology is clear: a delay between a physical input occurring (e.g., I/O pin voltage change) and it being processed electronically (I/O pins read by processor and memory registers updated to reflect the state of the pins) is input lag, and a delay between an electronic output being sent (e.g., memory register set to reflect the desired state of an output) and it being processed into a physically observable phenomenon (e.g., memory register read and I/O pin voltage modified accordingly), is output lag. In short, input lag occurs on input devices while output lag occurs on output devices.
The following are descriptions based on the colloquial use of the term, as used by gamers.
The potential causes for "input lag"- according to the second definition- are described below (steps which have negligible contributions to the input lag have been omitted). Each step in the process increases "input lag", however the net result may be unnoticeable if the overall "input lag" is low enough.
Controller sends signal to console
For wired controllers, this lag is negligible. For wireless controllers, opinions vary as to the significance of this lag. Some people claim to notice extra lag when using a wireless controller, while other people claim that the 4–8 milliseconds of lag is negligible.
Console/PC processes next frame
A videogame console or PC will send out a new frame once it has finished performing the necessary calculations to create it. The amount of frames rendered per second (on average) is called the frame rate. Using common 60 Hz monitor as an example, the maximum theoretical frame rate is 60 FPS (frames per second), which means the minimum theoretical input lag for the overall system is approximately 16.67 ms (1 frame/60 FPS). The monitor is usually the bottleneck for the theoretical maximum FPS, since there is no point in rendering more frames than the monitor can display per second. In situations where the CPU and/or GPU load is high, FPS can drop below the monitor's refresh rate.
Individual frames need not be finished within the interval of a screen refresh to output at an equivalent rate. Game engines often make use of pipelining architectures to process multiple frames concurrently, allowing for a more efficient use of the underlying hardware. This exacerbates input lag, especially at low frame rates.
This is the lag caused by the television or monitor (which is also called "input lag" by the first definition above, but "output lag' by the second definition). Image processing (such as upscaling, motion smoothing, or edge smoothing) takes time and therefore adds some degree of input lag. An input lag below 30 ms is generally considered unnoticeable in a television. Once the frame has been processed, the final step is the updating the pixels to display the correct color for the new frame. The time this takes is called the pixel response time.
Typical overall response times
Testing has found that overall "input lag" (from controller input to display response) times of approximately 200 ms are distracting to the user. It also appears that (excluding the monitor/television display lag) 133 ms is an average response time and the most sensitive games (fighting games, first person shooters and rhythm games) achieve response times of 67 ms (excluding display lag).
Input Lag Test: TVs from 2016 + 2017 Dein-Fernseher.de
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